1. Field of the Invention
The present invention relates to an output voltage detecting circuit for a DC-DC convertor.
2. Description of the Related Art
As a conventional step-up type DC-DC converter, there has been known a circuit shown in FIG. 3. That is, the circuit is made up of an error amplifier 101, a reference voltage source 102, a chopping wave generating circuit 103, a voltage comparator 104, a P-MOS transistor 105 functioning as a switching element, a Schottky diode 106, a voltage dividing circuit 110 formed of a resistor 108 and a resistor 109, and a smoothing circuit formed of a coil 107 and a capacitor 111. A difference between a voltage at a terminal B resulting from dividing a voltage VOUT of an output terminal OUT by a voltage dividing circuit 110 and a voltage at a terminal A which is an output of the reference voltage source 102 is amplified by the error amplifier 101. An output voltage of the error amplifier 101 and a chopping wave output by the chopping wave generating circuit 103 are compared with each other by the voltage comparator 104, and the P-MOS transistor 105 functioning as the switching element is rendered conductive or nonconductive to obtain a constant regulated output voltage VOUT at the output terminal OUT.
In the case where the amplification degree of the error amplifier 101 is sufficiently large, and assuming that the resistance value of the resistor 108 is R108, the resistance value of the resistor 109 is R109 and the voltage at the terminal A which is the output of the reference voltage source 102 is VREF, the voltage VOUT at the output terminal OUT is controlled so as to satisfy Expression (1).
VOUT=(1+R108/R109)xc3x97VREFxe2x80x83xe2x80x83(1)
The conventional step-up type DC-DC converter shown in FIG. 3 suffers from a drawback in that the output voltage VOUT is boosted to be higher than the desired regulated output voltage when a load rapidly changes or a ripple voltage abnormally increases. When the output voltage VOUT greatly exceeds the desired regulated output range voltage, since an excessive voltage is applied to the load, there arises the problem that an electronic circuit connected to the DC-DC converter as the load maybe destroyed.
The present invention has been made to solve the above problem with the conventional DC-DC converter, and therefore an object of the present invention is to provide an output voltage detecting circuit which is capable of preventing the output voltage from abnormally increasing beyond the desired regulated output voltage range.
In order to achieve the above object, according to the present invention, there is provided an output voltage detecting circuit that monitors a voltage VOUT at an output terminal in the DC-DC converter, and immediately stops the operation of the DC-DC converter to prevent the output from further increasing when the output voltage abnormally exceeds the desired regulated output range voltage.